Hi Kundan, if you don't know how to use an editing software like Photoshop, etc. you can still find a good image for your course at this site: https://pixabay.com/
Images there are almost all free to use... the only thing you'll have to change is the dimensions, as Udemy requires you to have 750 x 422 pixel.
Hope this can help.
whatever you do, do not just put up a picture from any stock site. There are so many courses here using generic stock... you want to create something that is unique, standouts and tells your story. You can also apply to Udemy for a free course graphic. This will not happen until after you submit your course. Make you you give plenty of detail. Personally I don’t use them as they don’t work for me however for others they are fabulous.
I like to create a course graphic early on whilst I am in course production as it works to inspire me, plus the graphic usually evolves with the course as it builds.